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Feasibility Study/Corrective Action Plan 8 <br /> The drake Property 26485 Thornton Road Thornton California November 26 1996 <br /> 1) There is no reduction in the source concentration over time This results in an over-estimation <br /> because if compounds are partitioning from the soil to water, then soil concentrations must <br /> decrease, <br /> 2) No natural bioattenuation or decay is occurring Greater concentrations would be expected in <br /> monitoring wells based on calculations if natural attenuation is not occurring, <br /> 3) The impacted soils are saturated at all times Historic water levels indicate that water is <br /> periodically deeper than impacted soils, and <br /> 4) Equilibrium between soil and water is instantaneous <br /> Comparison of Site Conditions With Risk-Based Screening Levels <br /> Table 6 presents the American Society for Testing and Materials' (ASTM's) Risk-Based Screening Levels <br /> (RBSLs) for BTEX based on exposure pathways for residential or commercial/industrial use of the site The <br /> RBSLs correspond to a potential cancer risk level of one additional cancer in 1 million cases of exposure, <br /> and a Hazard Index of 1 0 for noncancer health effects <br /> The results of the comparison to RBSLs demonstrate that the maximum detected concentrations in soil and <br /> groundwater do not exceed the risk-based screening levels for BTEX detected at the site The maximum <br /> theoretical groundwater concentrations are also below the RBSLs The ingestion of groundwater is not <br /> considered a complete exposure pathway for this site, but has been included for purposes of comparison <br /> This evaluation indicates that site concentrations of BTEX do not pose a threat to human health and the <br /> environment <br /> 60 CONCLUSIONS AND RECOMMENDATIONS <br /> ■ The former service station ceased operations and closed in 1975 The source of hydrocarbon <br /> contamination has been mitigated by the removal of all known USTs, pump islands and <br /> associated piping in 1975 and 1993 Subsequent to the 1993 UST removal, additional soils <br /> impacted with BTEX and TPH-G have been excavated from the site <br /> ■ The extent of hydrocarbon-impacted soils and groundwater has been defined through previous <br /> site assessments (Wallace-Kuhl 1993, 1994, Groundwater Technology, 1995) In addition, the <br /> station was closed in 1975, and all known USTs removed from the site in 1975 and 1993 <br /> ■ Remediation of hydrocarbon-impacted soils on site has been successfully achieved using the <br /> most cost-effective Best Available Technology (BAT) (i e , excavation of impacted soils), as <br /> demonstrated by non-detectable benzene levels, and the general trend of diminishing <br /> ethylbenzene toluene and total xylenes concentrations over time In addition soil and <br /> groundwater collected from soil borings advanced in 1995 did not contain detectable levels of <br /> 0191FEn RPT(Mrsc 15) <br /> FLUOR DANIEL GTI <br />